There are several research territories which contribute to the prior art of the present invention. They include liquid crystal electro-optic modulation, the dielectric properties of liquid crystals, second-order nonlinear optical effects, and prism-coupled waveguides. Each is discussed below.
Liquid crystal electro-optic modulation has been utilized in a number of device applications. Nematic liquid crystals provide analog retardation changes due to rotation of birefringent molecules out of the plane of the incident optical field with tuning speeds of 1-100 ms. Chiral smectic liquid crystals (CSLCs) provide tuning speeds of 1 .mu.s. When incorporated in a "bookshelf" geometry cell (smectic layers oriented perpendicular to the substrate walls), analog CSLC materials, such as SmA* (S. T. Lagerwall et al. in U.S. Pat. No. 4,838,663) and distorted helix ferroelectrics, DHF (L. A. Beresnev et al., European Patent Application No. 309774, published 1989), display an analog tilt of the cell optical axis in the plane of the cell walls upon application of an electric field across the smectic layers. In a discrete, multi-state cell, for example using ferroelectric SmC* or SmH* (N. A. Clark et al. in U.S. Pat. No. 4,367,924 and U.S. Pat. No. 4,563,059) or antiferroelectric phases (see for example, I. Nishiyama et al., Jpn. J. Appl. Phys. 28, L2248 (1989)), application of an electric field above a certain threshold voltage results in switching of the tilt of the CSLC molecules between discrete stable states. In homeotropically aligned cells (smectic layers parallel to substrate walls) the optical axis of the CSLC material rotates in a plane perpendicular to the cell walls on application of an electric field across the smectic layers by electrodes that are lateral to the substrate walls (Sharp et al., U.S. patent application 07/792,284 filed Nov. 14, 1991 and S. Garoff et al. Phys. Rev. Lett. 38, 848 (1977)). The devices of the prior art of nematic liquid crystal cell and CSLCs all function by large molecular reorientation of the molecular director (a vector on the long molecular axis).
The dielectric properties of liquid crystals have been characterized by a number of groups (L. Bata et al., in Advances in Liquid Crystal Research and Applications, edited by L. Bata, Pergamon Press (Oxford) 1980, p. 251; A. Buka, ibid. p. 261; N. Maruyama, Ferroelectrics 58, 187 (1984); and F. Gouda et al., Ferroelectrics 113, 165 (1991)). The modes utilized in the prior art of liquid crystal devices, which are associated with reorientation of the molecular director, are identified in CSLCs as the Goldstone mode (azimuthal fluctuation of the molecular director) and the "soft" mode (fluctuation of the tilt angle). Typical frequencies for these modes are 10-100 Hz and 10.sup.4 -10.sup.5 Hz respectively. Both modes are present in SmC* materials and only the soft mode is present in SmA* materials. Other contributions to the dielectric permittivity are identified as rotation about the molecular short axis, reorientation about the short-axis, rotation about the long axis, and intramolecular rotation about a single bond. These modes occur at higher frequencies (10.sup.6 -10.sup.12 Hz) than the molecular reorientation modes. At still higher frequency is electronic oscillation.
Optical second-order nonlinear (X.sup.2) effects include second harmonic generation (SHG), the linear electro-optic (EO) effect (Pockels effect), parametric amplification, optical rectification, and frequency mixing (see, for example, A. Yariv, Optical Electronics 3rd ed., CBS College Publisher, 1985). The origin of these nonlinear processes is field-induced displacement of the centers of positive and negative charged matter. In the linear EO effect, an applied electric field alters the index of refraction of the medium. This affords a convenient and widely used means of modulating the phase or intensity of optical radiation. Applications of the linear EO effect include optical modulation, spectral filters, and beam deflectors.
In the prior art, second-order nonlinear optical effects have been demonstrated in inorganic crystals and in organic materials such as Langmuir-Blodgett films, polymeric solid solutions, main chain polymers, and side chain polymers (T. Kondo et al., Jpn. J. Appl. Phys. 28, 1622 (1989) and D. Jungbaver et al., J. Appl. Phys. 69, 8011 (1991)). Chiral smectic liquid crystals in the thermodynamically stable smectic C* phase have C.sub.2 polar symmetry and possess macroscopic order, the requirements for displaying nonlinear optical effects. The inventors have previously demonstrated second harmonic generation with CSLCs (J. Y. Liu et al., Opt. Lett. 15, 267 (1990) and J. Y. Liu, Ph.D. Thesis, University of Colorado, 1992). In addition, discovering the molecular criteria for CSLCs with increased nonlinear optical response and synthesizing such compounds has been undertaken (D. M. Walba et al., Mol. Cryst. Liq. Cryst. 198, 51 (1991) and J. Am. Chem Soc. 113, 5471 (1991)).
A technique is presented in the prior art for sensitive measurements of dielectric materials using prism-coupled thin-film waveguides (P. K. Tien et al., J. Opt. Soc. Am. 60, 1325 (1970) and R. Ulrich et al., Appl. Opt. 12, 2901 (1973)). Prism-coupling provides an efficient method of coupling a light beam into a thin film waveguide. In this technique a high refractive index prism is placed above a thin-film waveguide, separated by a low index cladding layer. For efficient coupling, the components of the wave vectors parallel to the gap are equal in the prism and the waveguide. This device has been demonstrated with inorganic thin-films.
Waveguides have been constructed with both nematic and smectic liquid crystal materials (M. Kobayashi et al., IEEE J. Quantum Elect. OE-18, 1603, 1982; T. G. Giallorenzi et al., J. Appl. Phys. 47, 1820, 1976; and S. S. Bawa et al., Appl. Phys. lett. 57, 1479, 1990). These devices utilize refractive index changes due to molecular reorientation to provide phase retardation. Some of these devices use prisms to couple monochromatic incident light into a polymer waveguide, which in turn is coupled into a liquid crystal waveguide. This technique is distinct from prism coupling directly into the waveguide in that modulating the liquid crystal waveguide refractive index does not affect the coupling condition at the prism-polymer waveguide interface. Liquid crystals have also been used in tunable Fabry-Perot filters (see, for example, M. W. Maeda et al. IEEE Photonics Tech. Lett. 2, 820 (1990) and A. Miller et al., U.S. Pat. No. 4,790,634) but, as in the case of liquid crystal waveguides, they utilize molecular reorientation for modulation.